This proposed research involves a study of the biological effect of thyroid hormone on cultured cardiac cells prepared from the newborn rat heart. These cells can be grown in media containing thyroidectomized calf serum and respond to physiological concentrations of thyroid hormone with regard to its effect on glucose utilization and cyclic AMP response to epinephrine. I propose to utilize this system to define the mechanism of action of thyroid hormones on the regulation of glucose metabolism, myosin synthesis and an interrelationship with catecholamines. It is hoped that this study will further our understanding of the action of thyroid hormone of the heart "in vivo."